High-frequency discharge pumped axial-flow CO.sub.2 lasers are in the process of being developed because of their advantages of high-power output and stable oscillation. One example of such laser is disclosed in Japanese Patent Application No. 61-243212 filed by the applicant.
A conventional axial-flow-type high-frequency discharge pumped laser oscillator device is illustrated in FIG. 10 of the accompanying drawings. The illustrated laser oscillator device includes a discharge tube 1 comprising four tube segments. However, the discharge tube 1 may have a desired number of tube segments dependent on the output to be produced. A total reflection mirror 2 and an output coupling mirror 3 are accurately positioned at the ends of the discharge tube 1. Denoted at 4 is an outgoing laser beam. The segments of the discharge tube 1 have gas inlet and outlet ports connected to a single roots blower 7. Cooling units 5, 6 serve to cool a laser gas heated by the discharge and the roots blower 7. The laser gas flows in the discharge tube 1 and gas delivery tubes in the directions of the arrows. Electrodes 8a, 8b.about.11a, 11b are connected to respective high-frequency power supplies 12, 13, 14, 15. The gas flows in the discharge tube 1 at the speed of about 100 m/second. An electric discharge is produced in the discharge tube 1 by a high-frequency voltage applied by the high-frequency power supplies 12.about.15 to generate laser oscillation.
The principles of a conventional high-frequency power supply are illustrated in FIG. 5 of the accompanying drawings. Denoted at 16 is a DC power supply, and 17 a high-frequency (RF) power supply. DC output from the DC power supply 16 is applied to the high-frequency power supply 17. The high-frequency power supply 17 comprises a branch circuit comprising four FETs 18.about.21, a boost transformer 22, and an impedance matching circuit 23. The output terminals of the high-frequency power supply 17 are coupled to the discharge tube 1 through electrodes 8a1, 8a2. A CT coil 24 serves to detect a current flowing through the discharge tube 1. A current feedback line 25 feeds back the detected current to control the current supplied to the discharge tube 1 to be constant.
The high-frequency discharge pumping laser has the following many advantages over the ordinary DC discharge pumping laser:
(i) Since it produces an electrodeless discharge, there is no problem of electrode material deterioration.
(ii) No ballast resistor is required.
(iii) There is no cathode fall, and high energy efficiency is obtained.
(iv) The laser can be operated at a low voltage and hence is safe for the operator.
(v) The dissociation ratio of CO.sub.2, is low, and so is the running cost.
(vi) The laser has excellent pulse characteristics.
(vii) The laser device can be reduced in size.
(viii) The phenomenon of electron capture can be utilized by sufficiently increasing the frequency to obtain high efficiency.
(ix) The freedom of selection of discharge tube materials is high.
However, the current and voltage of the high-frequency discharge pumping laser tend to oscillate before and after a discharge is started, based on plasma coupling. Oscillation at about 1.about.10 Hz occurs when the discharge current in the discharge tube 1 is controlled by the feedback system shown in FIG. 5. As a result, various alarming conditions for the DC power supply 16 tend to take place, and the FETs 18.about.21 may be destroyed. In addition, the laser output is rendered unstable.